Fats and oils having high SFI (solid fat index) are effectively utilized by fractionation thereof into high and low melting point fractions. In general, high melting point fractions are more costly than low melting point fractions. In fact, for example, in case of palm kernel oil (PKO), since its high melting point fraction (PKS) is useful as a raw material for the production of a cocoa butter substitute (CBS) and the like, PKS is marketed at a higher price than a low melting point fraction (PKL) of PKO and PKL is even cheaper than PKO per se as a raw material for the fractionation. Then, for the fractionation of fats and oils having high SFI, in many cases, attempts have been made to increase the yields of high melting point fractions as high as possible efficiently.
At present, a representative process for the fractionation of palm kernel oil employed in the Malay Peninsula region is the dry fractionation, that is, fractionation of fats and oils without using any solvent, detergent or the like.
In the typical dry fractionation, PKO is pre-cooled to about 27.degree. C. and distributed into many trays, followed by allowing it to stand at 18 to 21.degree. C. for about 10 hours to crystallize, wrapping up the resultant cakes containing fat crystals with a filter cloth and subjecting the wrapped cakes to filtration under pressure (with a hydraulic press) to separate solids from a liquid phase ("SPECIALTY FATS VERSUS COCOA BUTTER" By Wong Soon, 1991). Hereinafter, this process is referred to as the conventional process.
In the conventional process, for increasing the yield of PKS, it is required to sufficiently carry out standing of pre-cooled PKO distributed into trays to increase the amount of fat crystals to be formed. On the other hand, this causes difficulties in filtration (separation of a liquid phase from solids) and, in order to recover fat crystals with good quality, it is necessary to press the cakes containing fat crystals under high pressure with a hydraulic press for a long period of time. However, there is a certain limit to an increase in the yield of PKS by this procedure. Then, the improvement of the conventional process is directed toward solving problems caused by intensive labor type steps as described below rather than to increase in the yield of PKS.
That is, the conventional process has been widely employed because of its low costs of facilities. However, a large number of trays are used in the standing step for crystallization (it is said that many as 10,000 to 20,000 trays are required for the facilities treating 100 ton of PKO per day). This step is very simple and trays are merely allowed to stand in a large room or space (to put trays on shelves). Then, un-uniform atmospheric temperatures of respective trays cannot be avoided and to control crystallization temperatures and time are difficult, which results in the problem that quality of products is apt to be inconsistent. In addition, there is such a defect that the filter cloth is apt to be worn out due to high pressure.
Furthermore, when the steps from the standing to the filtration under pressure are inspected in detail, various steps such as those for releasing the cakes containing fat crystals which are in the state of solid or semi-plastic from respective trays, wrapping them individually, transferring the wrapped cakes and then laying them up in a hydraulic press are required. However, these respective steps can hardly be automated and a great deal of labor is required. In fact, it is said that 70 to 80 persons are required for the facilities treating 100 ton of PKO per day. Therefore, from an economic viewpoint, the conventional process would no longer be realized except in such a region that considerably cheaper manpower is available.
If it were possible to transfer the cakes containing fat crystals after crystallization, an automatic filter press could be used instead of a hydraulic press because the slurry of cakes could be transferred into the filter press through a pipe line and filtered by the filter press. When a filter press can be used, such intensive labor type steps as wrapping the cakes with a filter cloth and laying it up in a hydraulic press can be eliminated. Then, some attempts have been made to employ a filter press. However, even if the cakes after full crystallization are crushed or smashed, a slurry having sufficient fluidity cannot be obtained and therefore the fatty material can hardly be transferred through a pipe line. Accordingly, at present, the amount of fat crystals to be formed is compelled to be controlled to maintain fluidability of slurry after crushing. That is, attempts for saving manpower are being made at the sacrifice of the yield of PKS.